Method for driving optical modulation device

ABSTRACT

A method for driving an optical modulation device of a group of scanning electrodes and a group of signal electrodes arranged so that picture elements are defined at the intersections therebetween and a bistable optical modulation material assuming one of two stable states between the groups of electrodes, by in a first phase orienting the bistable material at the picture elements on an N-th scanning electrode to one stable state, and applying a writing signal to the signal electrodes in synchronism with a scanning signal to the N-th scanning electrode and while orienting the bistable material at the picture elements on an N+1-th scanning electrode to the one stable state. Also disclosed is a driving mode wherein a scanning signal is applied to the whole or a part of the scanning electrodes while, in phase therewith, a signal is applied to the whole or a part of the signal electrodes so that the optical modulation material is oriented to a first stable state, and a second step in which a scanning signal is applied to the whole or a part of the scanning electrodes while in phase therewith, a signal is applied to a selected signal electrode among the whole or a part of the signal electrodes so that the bistable optical modulation material is oriented to a second stable state.

This application is a division of application Ser. No. 07/865,630 filedApr. 9, 1992, now U.S. Pat. No. 5,381,254, which is a continuation ofSer. No. 07/302,083 filed on Jan. 26, 1989 now abandoned, which is acontinuation of Ser. No. 07/139,130 filed on Dec. 28, 1987 nowabandoned, which is a continuation of Ser. No. 06/701,765 filed on Feb.14, 1985 now abandoned.

BACKGROUND OF THE INVENTION

The present invention relates to a method for driving an opticalmodulation device and more particularly a time-division or time-sharingdriving method for driving an optical modulation device such as adisplay element, an optical shutter array and the like and especiallyfor driving a ferroelectric liquid crystal device.

Liquid crystal display devices for displaying a pattern or informationhave been well known in which a plurality of scanning electrodes and aplurality of signal electrodes are arranged in the form of a matrix andliquid crystals are interposed between the scanning and signalelectrodes, whereby a plurality of picture elements ("pixel")aredefined. In order to drive such liquid crystal display devices, atime-division driving method is used in which an address signal isapplied sequentially and, periodically to the scanning electrodes and insynchronism with the address signal, predetermined information signalsare selectively applied to the signal electrodes in parallel. Suchliquid crystal display devices and the methods for driving them haveserious defects as will be described below.

A first defect is that it is difficult to increase the density ofpicture elements or the size of a picture. Liquid crystals which havebeen used in practice in liquid crystal display devices, because oftheir fast response and their low electric power consumption, aretwisted nematic liquic crystals of the kind disclosed in, for instance,"Voltage Dependent Optical Activity of a Twisted Nematic LiquidCrystals", M. Schadt and W. Helfrich in Applied Physics Letters, Vol.18, No. 4, (Feb. 15, 1971), pages 127-128. The liquid crystals of thiskind assume a helical structure in which the molecules of a nematicliquid crystal having a positive dielectric anisotropy are twisted inthe direction of the thickness of the crystal liquid and are arranged inparallel with each other between electrodes when an electric field isnot applied. When an electric field is applied, the molecules of thenematic liquid crystal with a positive dielectric anisotropy areoriented in the direction of the electric field, causing opticalmodulation. In the case of a display device in which a liquid crystal ofthe kind described is used and which has a matrix of electrodes, avoltage higher than a threshold voltage-required for arranging themolecules of the liquid crystal in the direction perpendicular thesurfaces of the electrodes is applied to a region (selected point) atwhich both a scanning electrode and a signal electrode are selected andno voltage is applied to a region (non-selected point) at which neithera scanning electrode nor a signal electrode is selected. As a result,the molecules of the liquid crystal are maintained in a stable state inwhich they are in parallel with the surfaces of the electrodes. Whenlinear polarizers are disposed upon the upper and lower surfaces of aliquid crystal cell or device of the type described in cross nicolrelationship, a selected point prevents the transmission of light whilea non-selected point permits the transmission of light, whereby adisplay or picture is formed. However, in the case of a liquid crystaldevice with a matrix of electrodes, a finite electric field is appliedto a region (the so-called "semi-selected or half-selected point") inwhich a scanning electrode is selected while a signal electrode is notselected or in which a scanning electrode is not selected while a signalelectrode is selected. When the difference between a voltage applied toa selected point and a voltage applied to a half-selected point issufficiently large, and if a threshold voltage at which the molecules ofa liquid crystal are oriented in the direction perpendicular to anelectric field applied is between the above described voltages, thecorrect operation of a display element can be ensured. However, when thenumber (N) of scanning lines is increased, a time period (duty ratio)during which one selected point is subjected to an effective electricfield during the time when one frame is scanned is decreased at a ratioof 1/N. As a consequence, in the case of repetitive scanning, thegreater the number of scanning lines, the smaller the effective voltagedifference between a selected point and a non-selected point becomes. Asa result, the problems of reduction in contrast of a picture and ofcrosstalk are unavoidable. These essentially unavoidable problems resultwhen a driving method (that is, a repetitive scanning method) in which aliquid crystal which is not bistable (that is, a liquid crystal in whichthe molecules assume a stable state in which they are oriented in thehorizontal direction relative to the surfaces of the electrodes and areoriented in the vertical direction only when an effective electric fieldis applied) is driven by utilizing a time storage effect. In order toovercome these problems, there have been proposed a voltage averagingmethod, a two-frequency driving, a multiple matrix method and so on.However, neither of these is satisfactory in solving the above describedproblems. Therefore, it is impossible at present to provide a displaydevice with a large picture size and with a high density of pictureelements because it is impossible to increase the number of scanninglines.

Meanwhile, a laser beam printer (LBP) in which the electrical signalsrepresenting a pattern are applied in the form of a light pattern to anelectro-photographic sensitive member is most excellent as a means forobtaining a hard copy in response to the electrical input signals in thefield of printers from the viewpoint of the density of picture elementsand the copying speed. However, the laser beam printers have somedefects as follows:

1. First, they are large in size as a printer.

2. Second, they have moving parts such as a polygon scanner which aredriven at high speeds so that noise is produced and these moving partsmust be machined with a higher degree of dimensional accuracy.

In order to overcome the above and other problems, there has beenproposed the use of a liquid crystal shutter array which is a means forconverting electrical signals into optical signals. However, in the caseof generating the picture-element signals with a liquid crystal shutterarray, 2000 signal generators are needed in order to write thepicture-element signals in a length of 200 mm at a rate of 10 dots permillimeter. Furthermore it is required to apply independent signals tothese signal generators through respective lead wires. For thesereasons, it has been difficult to provide a liquid crystal shutterarray.

In order to overcome the above and other problems another attempt ismade to apply one line of image signals in a time sharing manner bysignal generators divided into a plurality of times. This method makesit possible to use a common electrode in order to apply a signal to aplurality of signal generators. As a result, the number of conductorscan be reduced remarkably. However, when a liquid crystal lackingbistability is used and when the number (N) of lines is increased, theON time of a signal is substantially reduced to 1/N. As a result, therearise the problems that the quantity of light incident on aphotosensitive member is decreased and that crosstalk occurs.

SUMMARY OF THE INVENTION

An object of the present invention is therefore to provide a novelmethod for driving an optical modulation device, especially aferroelectric liquid crystal device, which can substantially overcomethe above problems encountered in the prior art liquid crystal displaydevices and liquid crystal optical shutters.

Another object of the present invention is to provide a method fordriving an optical modulation device, especially a ferroelectric liquidcrystal device, with a fast response.

A further object of the present invention is to provide a method fordriving an optical modulation device especially a ferroelectric liquidcrystal device, with a high density of picture elements.

The above and other objects of the present invention can be attained bya method for driving an optical modulation device of the type in which agroup of scanning electrodes and a group of signal electrodes are soarranged that picture elements are defined at the intersections,respectively, between the scanning and signal electrodes, and bistableoptical modulation materials which are made to assume either of twostable states in response to an electric field applied are interposedbetween the group of scanning electrodes and the group of signalelectrodes, having a first phase in which a bistable opticalmodulation-material corresponding to a picture element on an N-thscanning electrode is made to assume a first stable state, a secondphase in which a writing signal is applied to the group of signalelectrodes in synchronism with a scanning signal applied to the N-thscanning electrode and a third phase in which a bistable opticalmodulation material corresponding to a picture element on an N+1-thscanning electrode is made to assume a first stable state or by a methodfor driving an optical modulation device of the type having a group ofscanning electrodes, a group of signal electrodes and bistable opticalmodulation materials which are made to assume either of two stablestates in response to an electric field applied and which are interposedbetween the group of scanning electrodes and the group of signalelectrodes, having a first step in which a scanning signal is applied tothe whole or some of the scanning electrodes while, in synchronism withthe scanning signal, a signal is applied to the whole or some of thesignal electrodes so that the optical modulation materials are made toassume a first stable state and a second step in which a scanning signalis applied to whole or some of the scanning electrodes while, insynchronism with the scanning signal, a signal is applied to the wholeor some of the selected signal electrodes so that the bistable opticalmodulation materials are made to assume a second stable state.

BRIEF DESCRIPTION OF THE DRAWINGS

FIGS. 1 and 2 are schematic perspective views illustrating the basicoperation principle of a liquid crystal device used in the presentinvention;

FIG. 3 is a schematic plan view of a matrix of electrodes used in thedriving method in accordance with the present invention;

FIGS. 4(a)-(f) are views showing waveforms of electrical signals appliedto the electrodes;

FIG. 5 (combination of FIGS. 5A and 5B) shows the waveforms of voltagesapplied in time series;

FIGS. 6(a)-(f) show the waveforms of electrical signals applied to theelectrodes in an another embodiment of the present invention;

FIG. 7 (combination of FIGS. 7A and 7B) shows the waveforms of voltagesapplied in time series in said another embodiment of the presentinvention;

FIGS. 8A(a)-(f) show the waveforms of electrical signals applied to theelectrodes in a further embodiment of the present invention;

FIGS. 8B(a)-(d) show the waveforms applied to pixels on the particularscanning electrode to which a selection signal is applied.

FIG. 9 (combination of FIGS. 9A and 9B) shows the waveforms of voltagesapplied in time series in said further embodiment of the presentinvention;

FIGS. 10A(a)-(f) show the waveforms of electrical signals applied to theelectrodes in a yet a further embodiment of the present invention;

FIGS. 10B(a)-(d) show the waveforms applied to pixels on the particularscanning electrode to which a selection signal is applied.

FIG. 11 (combination of FIGS. 11A and 11B) shows the waveforms ofvoltages applied in time series in a further embodiment of the presentinvention;

FIG. 12 is a schematic plan view of a matrix of electrodes of a liquidcrystal device driven by the method of the present invention;

FIGS. 13(a)-(d) show the waveforms of electrical signals;

FIG. 14 (combination of FIGS. 14A and 14B) shows the waveforms ofvoltages applied in time series;

FIG. 15 is a schematic plan view of a liquid crystal optical shutterwhich is driven by the method of the present invention;

FIG. 16 (combination of FIGS. 16A and 16B) shows the waveforms ofvoltages applied in time series in a still further embodiment of thepresent invention; and

FIG. 17(a-d) shows the waveforms of voltages applied in yet furtherembodiment of the present invention.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

In one preferred embodiment of the present invention, an opticalmodulation device comprising a group of scanning electrodes which aresequentially and periodically selected in response to a scanning signal,a group of signal electrodes which are disposed in opposed relationshipwith the scanning electrodes and which are selected in response to apredetermined information signal, and a bistable optical modulationmaterial interposed between the scanning and signal electrodes andadapted to assume either a first optically stable state or a secondstable state depending on an electric field applied thereto, can bedriven by applying to a selected scanning electrode an electrical signalwhich has a phase T⁰ with a voltage for producing an electric field inone direction so that the optical modulation material is oriented to thefirst stable state regardless of an electrical signal applied to asignal electrode, and a phase T for writing an information signal with avoltage adapted to assist the optical modulation material to be setoriented to the second stable state in response to the electrical signalapplied to the signal electrode or more preferably by applying anelectric signal of the type described above in which the phase Tcomprises an information signal phase T¹ and an auxiliary signal phaseT² in which an electrical signal with a voltage polarity opposite tothat of an electrical signal applied in the phase T¹ to the group ofsignal electrodes in response to a predetermined information.

As an optical modulation material used in a driving method according tothe present invention, a material which shows either a firsts opticallystable state or a second optically stable state depending upon anelectric field applied thereto, i.e., has bistability with respect tothe applied electric field, particularly a liquid crystal having theabove-mentioned property, may be used.

Preferable liquid crystals having bistability which can be used in thedriving method according to the present invention are chiral smectic C(SmC*)- or H (SmH*)-phase liquid crystals having ferroelectricity. Theseferroelectric liquid crystals are described in, e.g., "LE JOURNAL DEPHYSIQUE LETTERS" 36 (L-69), 1975 "Ferroelectric Liquid Crystals";"Applied Physics Letters" 36 (11) 1980, "Submicro Second BistableElectrooptic Switching in Liquid Crystals", "Solid State Physics" 16(141) , 1981 "Liquid Crystal", etc Ferroelectric liquid crystalsdisclosed in these publications may be used in the present invention.

More particularly, examples of ferroelectric liquid crystal compoundusable in the method according to the present invention includedecyloxybenzylidene-p'-amino-2-methylbutyl cinnamate (DOBAMBC),hexyloxy-benzylidene-p'-amino-2-chloropropyl cinnamate (HOBACPC),4-o-(2-methyl)-butylresorcilidene-4'-octylaniline (MBRA8), etc.

In addition to the SmC*- or SmH*-phase liquid crystals as describedabove, liquid crystals showing chiral smectic I phase (SmI*), J phase(SmJ*), G phase (StaG*), F phase (SmF*) or K phase (SmK*) may also beused in the present invention.

When a device is constituted using these materials, the device may besupported with a block of copper, etc., in which a heater is embedded inorder to realize a temperature condition where the liquid crystalcompounds assume a smectic phase.

Referring to FIG. 1, there is schematically shown an example of aferroelectric liquid crystal cell for explanation of the operationthereof. Reference numerals 11 and 11a denote base plates (glass plates)on which a transparent electrode of, e.g., In₂ O₃, SnO₂, ITO (Indium-TinOxide), etc., is disposed, respectively. A liquid crystal of an SmC*- orSmH*-phase in which liquid crystal molecular layers 12, are orientedperpendicular to surfaces of the glass plates is hermetically disposedtherebetween. A full line 13 shows liquid crystal molecules. Each liquidcrystal molecule 13 has a dipole moment (P⊥) 14 in a directionperpendicular to the axis thereof. When a voltage higher than a certainthreshold level is applied between electrodes formed on the base plates11 and 11a, a helical structure of the liquid crystal molecule 13 isloosened and unwound to change the alignment direction of respectiveliquid crystal molecules 13 so that the dipole moments (P⊥) 14 are alldirected in the direction of the electric field. The liquid crystalmolecules 13 have an elongated shape and show refractive anisotropybetween the long axis and the short axis thereof. Accordingly, it iseasily understood that when, for instance, polarizers arranged in across nicol relationship, i.e., with their polarizing directionscrossing each other, are disposed on the upper and the lower surfaces ofthe glass plates, the liquid crystal cell thus arranged functions as aliquid crystal optical modulation device, of which opticalcharacteristics vary depending upon the polarity of an applied voltage.Further, when the thickness of the liquid crystal cell is sufficientlythin (e.g., 1μ), the helical structure of the liquid crystal moleculesis loosened even in the absence of an electric field whereby the dipolemoment assumes either of the two states, i.e., P in an upper direction24 or Pa in a lower direction 24a as shown in FIG. 2. When electricfield E or Ea higher than a certain threshold level and different fromeach other in polarity as shown in FIG. 2 is applied to a cell havingthe above-mentioned characteristics, the dipole moment is directedeither in the upper direction 24 or in the lower direction 24a dependingon the vector of the electric field E or Ea. In correspondence withthis, the liquid crystal molecules are oriented in either of a firststable state 23 and a second stable state 23a.

When the above-mentioned ferroelectric liquid crystal is used as anoptical modulation element, it is possible to obtain two advantages.First is that the response speed is quite fast. Second is that theorientation of the liquid crystal shows bistability. The secondadvantage will be further explained, e.g., with reference to FIG. 2.When the electric field E is applied to the liquid crystal molecules,they are oriented in the first stable state 23. This state is keptstable even if the electric field is removed. On the other hand, whenthe electric field Ea of which direction is opposite to that of theelectric field E is applied thereto, the liquid crystal molecules areoriented to the second stable state 23a, whereby the directions ofmolecules are changed. This state is also kept stable even if theelectric field is removed. Further, as long as the magnitude of theelectric field E being applied is not above a certain threshold value,the liquid crystal molecules are placed in the respective orientationstates. In order to effectively realize high response speed andbistability, it is preferable that the thickness of the cell is as thinas possible and generally. 0.5 to 20μ, particularly 1 to 5μ. A liquidcrystal-electrooptical device having a matrix electrode structure inwhich the ferroelectric liquid crystal of this kind is used is proposed,e.g., in the specification of U.S. Pat. No. 4,367,924 by Clark andLagerwall.

A preferred embodiment of the driving method according to the presentinvention is explained with reference to FIG. 3.

FIG. 3 is a schematic view of a cell 31 with a matrix of electrodescomprising scanning electrodes 32 and signal electrodes 33, and aferroelectric liquid crystal interposed therebetween. For the sake ofbetter understanding of the present invention, a simple case in which apicture element becomes black or white in response to a binary signalwill be described. In FIG. 3, the hatched picture elements represent"black" while the white picture elements, "white". FIG. 4(a) shows anelectric signal applied to a selected scanning electrode; FIG. 4(b)shows an electrical signal applied to the other scanning electrodes(non-selected scanning electrode); and FIGS. 4(c) and (d) respectivelyshow electrical signals which are applied to a selected signal electrode(which represents "black"). More particularly, FIG. 4(c) shows anelectrical signal applied to a selected signal electrode when thepreceding electrical signal has represented "black", while FIG. 4(d)shows an electrical signal applied to a selected signal electrode whenthe preceding electrical signal has represented "white". FIGS. 4(e) and(f) show electrical signals which are applied to the signal electrodeswhich are not selected (and which represent "white"). More particularly,FIG. 4(e) shows an electrical signal applied to the non-selected signalelectrodes when the preceding electrical signal has represented "black"while FIG. 4(f) shows an electrical signal applied to the non-selectedsignal electrodes when the preceding electrical signal has represented"white". In a phase T⁰, all the picture elements on one scanningelectrode once become "white"; and in a phase T, an information signalis written. In this embodiment, T⁰ =T=Δt. FIG. 5 (combination of FIGS.5A and 5B) shows the driving waveforms when the pattern as shown in FIG.3 is displayed in response to the electrical signals as shown in FIG. 4.In FIG. 5, S₁ -S₅ represent the signals applied to the scanningelectrodes; I₁ and I₃, the signals applied to the signal electrodes I₁and I₃, respectively; and A and C, the waveforms of voltages applied tothe picture elements A and C, respectively, shown in FIG. 3. A₁threshold voltage when it is applied for a time period of Δt in order todrive a bistable liquid crystal into a first stable state (in which apicture element becomes "white") is represented by -Vth₂, while athreshold voltage when it is applied for a time period of Δt in order todrive the bistable liquid crystal into a second stable state (in which apicture element becomes "black") is represented by Vth₁. Then, the valueof V₀ is so selected that the following relations may be satisfied:

    V.sub.0 <Vth.sub.1 <2V.sub.0,

and

    -2V.sub.0 <-Vth.sub.2 <-V.sub.0.

As is clear from FIG. 5, all the picture elements on one scanningelectrode are caused to become "white" once and thereafter "black" or"white" is selected in response to information. In the case of a pictureelement which represents "black", the reversal from "white" to "black"occurs, whereby the information is written. When the writing ofinformation into the picture elements on one scanning electrode is beingcarried out within a predetermined phase (time period), the operationfor causing all the picture elements on the next scanning electrode tobecome "white" is simultaneously carried out. Therefore, the operationfor writing information into all the picture elements in one frame byscanning can be accomplished at a very high speed.

Another embodiment of the driving method in accordance with the presentinvention is shown in FIGS. 6 and 7. FIG. 6(a) shows an electric signalapplied to a selected scanning electrode; and FIG. 6(b) shows anelectric signal applied to the scanning electrodes which are notselected. FIGS. 6(c)-(f) show electrical signals applied to the signalelectrodes. FIGS. 6(c) and (e) show the information signals applied whenthe preceding signal has represented "black", while FIGS. 6(d) and (f)show the information signals applied when the preceding signal hasrepresented "white". In FIGS. 6(c) and (d) , an information signal V₀for representing "black" is shown as being applied in a phase T, whilean information signal -V₀ for representing "white" is shown as beingapplied in the phase T in FIGS. 6(e) and (f) .

FIG. 7 (combination of FIGS. 7A and 7B) shows the driving waveforms whenthe pattern as shown in FIG. 3 is displayed. In FIG. 7, S₁ -S₅ representthe signals applied to the scanning electrodes; I₁ and I₃ represent thesignals applied to the signal electrodes I₁ and I₃, respectively; and Aand C represent the waveforms of the voltages applied to the pictureelements A₁ and C, respectively, of the pattern shown in FIG. 3.

Microscopic mechanism of switching due to electric field of aferroelectric liquid crystal having bistability has not been fullyclarified. Generally speaking, however, the ferroelectric liquid crystalcan retain its stable state semi-permanently, if it has been switched ororiented to the stable state by application of a strong electric fieldfor a predetermined time and is left standing under absolutely noelectric field. However, when a reverse polarity of an electric field ACvoltage is applied to the liquid crystal for a long period of time, evenif the electric field is such a weak field (corresponding to a voltagebelow V_(th) in the previous example) that the stable state of theliquid crystal is not switched in a predetermined time for writing, theliquid crystal can change its stable state to the other one, wherebycorrect display or modulation of information cannot be accomplished. Wehave recognized that the liability of such switching or reversal oforiented states under a long term application of a weak electric fieldis affected by the material and roughness of a base plate contacting theliquid crystal and the kind of the liquid crystal, but have notclarified the effects quantitatively. We have confirmed a tendency thata monoaxial treatment of the base plate such as rubbing or oblique ortilt vapor deposition of SiO, etc., increases the liability of theabove-mentioned reversal of oriented states. The tendency is manifestedat a higher temperature compared to a lower temperature.

Anyway, in order to accomplish correct display or modulation ofinformation, it is advisable that one direction of electric field isprevented from being applied to the liquid crystal for a long time.

In a preferred embodiment of the driving method in accordance with thepresent invention, therefore, there is provided an auxiliary signalphase T² in order to prevent the continuous application of an electricfield in one direction as will be described in detail with reference toFIGS. 8 and 9 hereinafter.

FIGS. 8A(a)-8A(f) and 10A(c)-10A(f), S_(s) refers to a scanningselection signal applied to a particular scanning electrode and S_(N) toa scanning nonselection signal. D_(B) is an information signal forwriting "black" in a pixel on the particular scanning electrode andD_(W) is an information signal for writing "white" in a pixel on theparticular scanning electrode. Ax is an auxiliary signal accompanyingthe D_(B) or D_(W). Pre-Ax is an auxiliary signal applied during ascanning selection period for a scanning electrode which is selectedprior to the particular scanning electrode. IB₁ and IB₂ are data signalsfor writing "black" in pixels inclusively showing Pre-AX signals appliedto the pixels subsequent to Pre-D_(B) and Pre-D_(W), respectively,applied during a preceding period for writing "black" and "white" in thepixels. IW₁ and IW₂ are data signals for writing "white" in pixelsinclusively showing Pre-Ax signals applied to the pixels subsequent toPre-D_(B) and Pre-D_(W), respectively, applied during a preceding periodfor writing "black" and "white" in the pixels. Pre-D_(B) (not shown) isan information signal for writing "black" in the pixel concerned in thepreceding period. Pre-D_(W) (not shown) is an information signal forwriting "white" in the pixels concerned in the preceding period.

FIG. 8A(a) shows an electrical signal applied to a selected scanningelectrode; and FIG. 8A(b), an electrical signal applied to the scanningelectrodes which are not selected. As shown in FIGS. 8A(c)-(f) , duringa phase T², AC voltage signals with a polarity opposite to that of theinformation signal applied in the phase T¹ (corresponding to "black" inFIGS. 8(c) and (d), and "white" in FIGS. 8A(e) and (f)) are applied to asignal electrode. This will be described in more detail in conjunctionwith the display of the pattern as shown in FIG. 3. In the case of thedriving method without the phase T², the picture element A₁ becomes"black" in response to the scanning of the scanning electrode S₁, butthere arises a problem that the picture element A erratically becomes"white" because when the scanning electrodes S₂, S₃ and so on aresuccessively scanned, the electrical signal of -V₀ is continuouslyapplied to the signal electrode I₁ and hence to the picture element A.However, if an auxiliary signal phase T² is provided as described above,there arises no problem of crosstalk as is clear from the time serialsignals shown in FIG. 8.

FIGS. 8A(c) and (e) show the electrical signals applied when thepreceding signal has represented "black", while FIGS. 8(d) and (f) showthe electrical signals applied when the preceding signal has represented"white".

FIGS 8B(a)-(d) show the waveforms applied to pixels on the particularscanning electrode to which a selection signal is applied. For example,FIG. 8B(a) illustrates signals S_(S) and IB₁ applied to a particularpixel.

FIG. 9 (combination of FIGS. 9A and 9B) shows the driving waveforms usedto display the pattern as shown in FIG. 3. In FIG. 9, S₁ -S₅ representthe signals applied to the scanning electrodes; I₁ and I₃ represent thesignals applied to the signal electrodes I₁ and I₃, respectively; and A₁and C represent the waveforms of the voltages applied to the pictureelements A₁ and C, respectively, as shown in FIG. 3.

The following notes refer to FIG. 9:

1. A pixel A₁ is selected into "black".

2. AC voltage not changing the selected black state.

3. The pixel A₁ is erased into "white" when the one-polarity voltage isapplied to S₁. At this time, the pixels on S₁ are non-selectivelysupplied with a voltage of -3V₀ to be simultaneously erased into"white".

4. A pixel A₂ is selected in "white".

5. The pixel A₂ on a subsequently selected scanning electrode S₂, i.e.,during the period of selecting the display states of the pixels on S₁.At this time, the pixels on S₂ are supplied with a voltage of -2V₀ or-4V₀ (varying depending on whether the auxiliary signal is +V₀ or -V₀)respectively exceeding the threshold V_(th) to be simultaneously erasedinto "white".

6. A pixel A₃ is selected in white.

7. The pixels on S₃ including A₃ are simultaneously erased into "white".

A further embodiment of the driving method in accordance with thepresent invention will be described with reference to FIGS. 10 and 11.In this embodiment, V₀, Vth₁ and Vth₂ are so selected that the followingrelations may be satisfied:

    V.sub.0 <Vth.sub.1 <3V.sub.0,

and

    -3V.sub.0 <-Vth.sub.2 <-V.sub.0.

FIG. 10A(a) shows the electrical signal applied to a selected scanningelectrode; and FIG. 10(b), the electrical signal applied to the scanningelectrodes which are not selected.

Meanwhile, an optimum time interval of the auxiliary signal phase T² isdependent upon the magnitude of a voltage applied to a signal electrode.When a voltage with a polarity opposite to that of a voltage appliedduring the information signal phase T¹ is applied, it is preferred ingeneral that when a higher voltage is applied, the time period of thephase T² is shorter while when a lower voltage is applied, the timeperiod is longer. However, when the time period is long, it takes a longtime to scan the whole picture. As a result, it is preferable to set T²≦T¹.

FIGS. 10A(c)-(f) show the information signals applied to the signalelectrodes. FIGS. 10A(c) and (e) show the information signals appliedwhen the preceding signal has represented "black", while FIGS. 10A(d)and (f) show the information signals applied when the preceding signalhas represented "white". In FIGS. 10A(c) and (d) , an information signalV₀ for representing "black" is applied during the phase T¹, and, inFIGS. 10(e) and (f), an information signal V₀ for representing "white"is applied during the phase T¹.

FIGS. 10B(a)-(d) show the waveforms applied to pixels on the particularscanning electrode to which a selection signal is applied. For exampleFIG. 10B(a) illustrates signals S_(S) and IB₁ applied to a particularpixel.

FIG. 11 shows the driving waveforms used when the pattern as shown inFIG. 3 is displayed. In FIG. 11, S₁ -S₅ represent the signals applied tothe scanning electrodes; I₁ and I₃, the signals applied to theelectrodes I₁ and I₃, respectively; and A and C, the waveforms of thevoltages applied to the picture elements A and C, respectively, as shownin FIG. 3.

The present invention will now be explained with reference to workingexamples.

EXAMPLE 1

A pair of glass plates whose transparent conductor films (ITO) were sopatterned as to define a 500×5000 matrix were coated with a polyimidefilm of about 300 A in thickness by a spin coating process. Thereafterthe glass plates were subjected to a rubbing process with a roller aboutwhich a suede cloth was wound and then were joined together in such away that the rubbing directions were aligned, whereby a cell wasprovided. The cell gap was about 1.2μ. DOBAMBC, which is a ferroelectricliquid crystal, was filled into the cell and was gradually cooled fromthe heated and molten state, whereby a uniform monodomain in the SmCstate was obtained. The cell temperature was maintained at 70° C. and V₀was set to 10 V while the phases T⁰ =T¹ =T² =Δt were set to 50microseconds in Accordance with the driving method described above withreference to FIG. 10. Extremely high-quality pictures could be obtainedby the line-by-line scanning.

Yet another embodiment of the driving method in accordance with thepresent invention will be described with reference to FIG. 12 showingschematically a cell 121 with a matrix of electrodes and a ferroelectricliquid crystal (not shown) sandwiched between the electrodes. The cell121 has a group of scanning electrodes 122 and a group of signalelectrodes 123.

FIG. 13(a) shows the scanning signal applied to a selected scanningelectrode, and FIG. 13(b) the scanning signal applied to the scanningelectrodes which are not selected. FIG. 13(c) shows an electrical signal(referred to as a "white" signal) which drives a ferroelectric liquidcrystal into a first stable state and FIG. 13(d) shows an electricalsignal (referred to as a "black" signal) which drives the liquid crystalinto a second stable state.

First, as shown in FIG. 14 (combination of FIGS. 14A and 14B) in a firstframe F₁, the scanning signal is applied to the whole or a part of thescanning electrodes 122 and, in synchronism with the scanning signal,the "white" signal is applied to the whole or a part of the signalelectrodes 123. In a second frame F₂, the "black" signal is applied tothe predetermined portions as shown in FIG. 12 (black picture elements).FIG. 14 shows the waveforms of the voltages applied to the pictureelements A and B, respectively, as shown in FIG. 12 and the electricalsignals applied to the scanning electrodes 1221, 1222, 1223, 1224 and1225 and to the signal electrodes 1231, 1232, 1233, 1234 and 1235.

V⁰ is so selected that the following relations may be satisfied:

    V.sub.0 <Vth.sub.1 <2V.sub.0,

and

    -V.sub.0 >-Vth.sub.2 >2V.sub.0.

Therefore, as is clear from FIG. 14, -2V₀ is applied during the phase t₁to the whole picture elements on the scanning electrode to which thescanning signal is applied (or to the picture elements to be rewrittenin the case of rewriting) so that the ferroelectric liquid crystal isdriven into the first stable state. During the phase t₂, the voltageapplied to the picture elements is V₀, but V₀ <Vth₁ so that the firststate ("white") into which the liquid crystal has been driven during thephase t₁ can be maintained. As described above, during the first frameall the picture elements are once erased to "white" in response to the"white" signal. Thereafter during the second frame F₂ the "black" signalwhich is in synchronism with the scanning signal is applied to thesignal electrodes so that only the predetermined picture elements become"black". Thus one black-and-white picture is displayed. In this case,2V₀ is applied during the phase t₂ a to the picture element to which the"black" signal is applied after -2V₀ has .been applied during the phaset₁ a. As a result, the ferroelectric liquid crystal which stays in thefirst stable state during the phase t₁ a is caused to be driven into thesecond stable state during the phase t₂ a and consequently becomes"black".

The voltage V and the phase T (=t₁ +t₂) are dependent upon a liquid,crystal used and the thickness of a cell, but in general the voltage is3-70 V while the phase is in a range between 0.1 microsecond and 2milliseconds.

It would be apparent to those skilled in the art that in order toeffectively carry out the driving method in accordance with the presentinvention, the electrical signals applied to the scanning and signalelectrodes are not limited to simple signals having rectangularwaveforms as shown in FIG. 14 and that the driving method of the presentinvention can be carried out with signals having sinusoidal ortriangular wave forms.

FIG. 15 shows a matrix of electrodes of a liquid crystal optical shutterwhich operates based upon the driving method in accordance with thepresent invention. The optical shutter has a plurality of pictureelements 151 each with opposed transparent electrodes, a group ofscanning electrodes 152 and a group of signal electrodes 153.

FIG. 16 is a view used to explain a further embodiment of the drivingmethod in accordance with the present invention. In this embodiment, ascanning signal is sequentially applied to the scanning electrodes 122as shown in FIG. 12 and, in synchronism with the scanning signal, a"white" signal is applied to the signal electrodes 123, so that thewhole picture once becomes "white" during a first frame F₁. In thiscase, -2V₀ is applied to each picture element during the phase t₁ andthen V₀ which is lower than Vth₁ is applied during the phase t₂.Accordingly, the ferroelectric liquid crystal is driven into andmaintained in the first stable state during the phases t₁ and t₂.Thereafter, a "black" signal is applied only to predetermined pictureelements during a second frame F₂. The picture element (the blackpicture element shown in FIG. 12) to which the "black" signal is to beapplied with -2V₀ during a phase t₁ a and then with 2V₀ during a phaset₂ a. As a result, the ferroelectric liquid crystal in the pictureelement is driven into the second stable state. The picture element B isapplied with -V₀ and V₀, but V₀ satisfies the following relations asdescribed before: V₀ <V_(th1) <2V₀, and -V₀ >Vth₂ >-2V₀, so that thepicture element B will not be reversed to "white".

So far, the liquid crystal of a picture element has been described asbeing uniform and the whole region of each picture element has beendescribed as being driven into the first or second stable state.However, the orientation of a ferroelectric liquid crystal is influencedin an extremely delicate manner by the interaction between the liquidcrystal and the surfaces of the base plates. Accordingly, when thedifference between an applied voltage and a threshold voltage Vth₁ orVth₂ is small, a picture element can be driven into a state in whichsome molecules of the liquid crystal of the picture element are driveninto the first stable state while the remaining molecules into thesecond stable state. Therefore, it becomes possible by utilizing thisphenomenon to apply a signal during the second phase of an informationsignal so that a gradation of a picture element can be produced. Forinstance, when the same scanning signals are applied as in the case ofthe driving method described with reference to FIG. 14 or 16, it becomespossible to display a picture with a gradation by varying the number ofpulses of an information signal applied to the signal electrode duringthe phase t₂ a as shown in FIGS. 17(a)-(d).

Further, it is possible to utilize not only the variations in surfacecondition of the base plates which are the natural results of thetreatment of the base plates but also the conditions of the surfaceswith extremely fine mosaic patterns of the base plates.

The driving method in accordance with the present invention can beapplied in various fields such as liquid crystal optical shutters,liquid crystal television receivers, display devices and so on.

What is claimed is:
 1. A liquid crystal apparatus, comprising: aferroelectric liquid crystal device having a group of scanningelectrodes arranged in a matrix with and spaced apart from a group ofsignal electrodes with a ferroelectric liquid crystal disposedtherebetween so as to provide a picture element at each intersection ofthe scanning electrodes and the signal electrodes, and signalapplication means for applying information signals to the signalelectrodes in phase with scanning signals selectively applied to thescanning electrodes, said signal application means being arranged:(a) toapply a scanning selection signal comprising a former voltage of onepolarity and a latter voltage of the other polarity to a particular oneof the scanning electrodes to select that particular scanning electrode,and in synchronism with the scanning selection signal applied to selectthe particular scanning electrode, and to apply data signals to thesignal electrodes so that the pixels on the particular scanningelectrode supplied with the former voltage of one polarity arenon-selectively erased into one display state and the pixels on theparticular scanning electrode supplied with the latter voltage of theother polarity are respectively selected in display states depending onthe information signals applied in synchronism with the latter voltagesof the other polarity, the voltage polarities being determined withrespect to the voltage level of a scanning electrode to which thescanning selection signal is not applied; (b) to apply the formervoltage of one polarity of a subsequent scanning selection signal to ascanning electrode selected subsequent to said particular scanningelectrode during the period of applying the data signals for selectingthe display states of the pixels on said particular scanning electrode;and (c) in a period when the scanning electrode for that picture elementbecomes a non-selected scanning electrode, (i) to apply a voltage of onepolarity to the picture element, and (ii) before the application time ofsaid voltage of one polarity reaches a length of time beyond which saidvoltage of one polarity causes the inversion of the orientation stateinto another orientation state of the ferroelectric liquid crystal, toapply to the signal electrode corresponding to that picture element avoltage signal providing a zero voltage or a voltage of a polarityopposite to said one polarity to that picture element.
 2. An apparatusaccording to claim 1, wherein said voltage of one polarity and said zerovoltage or said voltage of the other polarity alternate with time.
 3. Anapparatus according to claim 1, wherein the voltage signal providingsaid zero voltage or said voltage of the other polarity is applied to asignal electrode before or after the application of the informationsignal to the signal electrode.
 4. An apparatus according to claim 1,wherein each scanning signal for selecting one of the scanningelectrodes is applied for a period of 0.1 usec to 2 msec.
 5. Anapparatus according to claim 1, wherein the scanning signals forselecting the scanning electrodes are applied periodically.
 6. Anapparatus according to claim 1, wherein the application period of saidzero voltage or said voltage of the other polarity is equal to orshorter than the application period of the scanning signals.
 7. Anapparatus according to claim 1, wherein the selected scanning electrodeor the signal electrode corresponding to the picture element where thestable state of the ferroelectric liquid crystal is provided is suppliedwith a signal having an asymmetric rectangular voltage waveform of onepolarity and the other polarity with respect to the voltage level of anon-selected scanning electrode.
 8. An apparatus according to claim 1,wherein said ferroelectric liquid crystal is a chiral smectic liquidcrystal.
 9. An apparatus according to claim 8, wherein said chiralsmectic liquid crystal is in chiral smectic C phase or H phase.
 10. Anapparatus according to claim 8, wherein said chiral smectic liquidcrystal is disposed in a layer thin enough to release its own helicalstructure.